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Old 11-10-2014, 11:01 AM   #1
armandozx
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R.A Carbon 110 roller conversion

Guys, quick run down on a recent tinkering project.

RA Carbon barrel 110cm
Pathos D'Angelo handle with reverse trigger mech with 28 grams of lead inside handle
Salvimar shaft 6.75 for 100cm gun
R.A reel with 50min of mares 2.0 line
MVD roller head with 18mm acceptable bands
16mm bands cut to 27in

Have not yet tested the gun, Dying to get out there and start using it. One thing I am debating is drilling into the barrel to anchor the muzzle, I instead used epoxy to secure the muzzle to the barrel. Reason being, the R'A's barrels are light carbon, I don't want them to start cracking when the holes are too close to the barrels end. Fiberglass was added to the barrel for a perfect pathos to rob allen fit, sanding will be necessary! Laid glass on the barrel for extra strength as oppose to the handle.

Cannot put photos for some reason, will update later on.
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Last edited by armandozx; 11-10-2014 at 05:17 PM.
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Old 11-10-2014, 12:25 PM   #2
SpearoTastic
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Re: R.A Carbon 110 roller conversion

A roller gun has the effective band stretch of a speargun ~25cm - 30cm longer than whatever barrel length you are using.

If you want to have the same penetration at distance as the longer speargun, use a shaft with the same mass as the shaft of the longer speargun.

For example, you are using a 110 cm barrel roller gun which should give you performance similar to a 140 cm conventional speargun with a single 18mm band with the same stretch ratio.

Using 40cm as the typical shaft overhang, the shaft mass for your 6.75mm x 140cm shaft is ~390 grams. However, a 140cm speargun is going to use a minimum 7mm diameter shaft. The shaft mass for a 7mm x 180cm shaft is ~540 grams. That is a difference of 150 grams.

To make sure the shaft behaves the same with the additional energy, it needs to have the same mass, which means you should use a 7.5mm x 150cm shaft (~520 grams) or an 8mm x 140cm shaft (~530 grams). Even though these shafts are not an exact match to the mass of the longer shaft, they are close enough and will give downrange performance far beyond the 6.75mm x 140cm shaft.

If you stick with the lighter shaft, you will gain muzzle velocity, but lose penetration at distance because the lighter shaft looses velocity quicker than the heavier shaft due to hydrodynamic frictional forces.

================

The other thing you will have to sort out is the amount of preload to use.

Preloading or pretensioning the band allows the band to provide a force on the shaft throughout the entire powerstroke, much like a pneumatic.

For example, assuming the bands are cut at 3.5:1 stretch ratio, we will first look at the roller without any preload. When the band comes to rest the force on shaft up to the moment before it is arrested is zero. The amount of energy placed into the shaft is (711 Newtons x 1.1 meters)/2 = 391 joules.

Now, if the bands are pretensioned by 100% elongation, the force on the shaft by the band the moment before it is arrested is ~332 N. So, the amount of energy placed into the shaft is [(711 N - 332 N) x 1.1 meters]/2 + 332 N x 1.1 meters = 573 joules..

So, over the same distance, the pretensioned setup provides an additoinal 182 joules of energy.

Please note, that the correct amount of pretension can be very shaft specific, so it varies from shaft to shaft, with larger diameter shafts handling more pretension than smaller diameter shafts, lengths being equal.

I have not come across a shortcut for determining the correct pretension, but have seen the 100% elongation recommended. The best advice I was given was to shorten the cord connecting the band ends to the speargun 1cm at a time until accuracy and/or penetration falls off, then lengthen the cord back to the setting that gave the most accuracy and/or penetration. Tedious, at best, but if you take good notes, you will know where to start on the next roller gun....
__________________

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Old 11-10-2014, 04:53 PM   #3
armandozx
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Re: R.A Carbon 110 roller conversion

Something worth considering ^ all of it and I will have to get into the mathematics of it at a later time. For starters the barrel is not a complete 110cm barrel and I shall measure for the specifics, I personally don't like anything above 8in of overhang on any of my shafts, having a reverse trigger mech sure eats some of that up which in this case comes in handy. I have a longer shaft RA, but the only lacks tabs and a resting tab as oppose to my shorter salvi. .25mm in diameter increase with aprox +/- 7in sure adds a bit perhaps not much to the performance of the shot but that will remain to be seen.

My goals now are the following test the setup and fix as needed. My load on the rest tab is at a 100% I have about 7 3/8in to pull to the closes tab towards the trigger mech which would exceed 100% band stretch.

If the loading is not as troublesome as I am expecting, I would adjust at the wishbone level, not the muzzle but at the bottom where they loop on the handle until things start getting somewhat uncomfortable. working on the photos as we speak, thanks for the write up, I appreciate it!
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Old 12-21-2014, 02:43 AM   #4
mishu1984
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Re: R.A Carbon 110 roller conversion

Quote:
Originally Posted by SpearoTastic View Post
A roller gun has the effective band stretch of a speargun ~25cm - 30cm longer than whatever barrel length you are using.

If you want to have the same penetration at distance as the longer speargun, use a shaft with the same mass as the shaft of the longer speargun.

For example, you are using a 110 cm barrel roller gun which should give you performance similar to a 140 cm conventional speargun with a single 18mm band with the same stretch ratio.

Using 40cm as the typical shaft overhang, the shaft mass for your 6.75mm x 140cm shaft is ~390 grams. However, a 140cm speargun is going to use a minimum 7mm diameter shaft. The shaft mass for a 7mm x 180cm shaft is ~540 grams. That is a difference of 150 grams.

To make sure the shaft behaves the same with the additional energy, it needs to have the same mass, which means you should use a 7.5mm x 150cm shaft (~520 grams) or an 8mm x 140cm shaft (~530 grams). Even though these shafts are not an exact match to the mass of the longer shaft, they are close enough and will give downrange performance far beyond the 6.75mm x 140cm shaft.

If you stick with the lighter shaft, you will gain muzzle velocity, but lose penetration at distance because the lighter shaft looses velocity quicker than the heavier shaft due to hydrodynamic frictional forces.

================

The other thing you will have to sort out is the amount of preload to use.

Preloading or pretensioning the band allows the band to provide a force on the shaft throughout the entire powerstroke, much like a pneumatic.

For example, assuming the bands are cut at 3.5:1 stretch ratio, we will first look at the roller without any preload. When the band comes to rest the force on shaft up to the moment before it is arrested is zero. The amount of energy placed into the shaft is (711 Newtons x 1.1 meters)/2 = 391 joules.

Now, if the bands are pretensioned by 100% elongation, the force on the shaft by the band the moment before it is arrested is ~332 N. So, the amount of energy placed into the shaft is [(711 N - 332 N) x 1.1 meters]/2 + 332 N x 1.1 meters = 573 joules..

So, over the same distance, the pretensioned setup provides an additoinal 182 joules of energy.

Please note, that the correct amount of pretension can be very shaft specific, so it varies from shaft to shaft, with larger diameter shafts handling more pretension than smaller diameter shafts, lengths being equal.

I have not come across a shortcut for determining the correct pretension, but have seen the 100% elongation recommended. The best advice I was given was to shorten the cord connecting the band ends to the speargun 1cm at a time until accuracy and/or penetration falls off, then lengthen the cord back to the setting that gave the most accuracy and/or penetration. Tedious, at best, but if you take good notes, you will know where to start on the next roller gun....
Does the band diameter play a role in pretension levels?
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Old 12-22-2014, 07:50 AM   #5
SpearoTastic
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Re: R.A Carbon 110 roller conversion

Quote:
Originally Posted by mishu1984 View Post
Does the band diameter play a role in pretension levels?

Band area is directly proportional to band force, so, yes, band diameter does play a role in pre-tension levels.

Pre-tensioning factors are:

How much you can pull back? If you cannot pull 60-80 lbs from the very top of your speargun, then do not use that much pre-tension.

How much your muzzle can handle? Too much pre-tension and you can cause your muzzle to fail. Remember, the muzzle has to arrest the bands at the pre-tension force, so, to the muzzle, an 80 lb pre-tension is like an 80 lb impact at the end of the powerstroke. Not all muzzles are designed for such a force.

The ratio of the shaft mass/band mass/speargun mass. Pre-tensioning is more critical with a thinner diameter shaft. One can expect the pre-tensioning force to increase with the diameter and mass of the shaft. For example, if you were to take an 80 cm railgun and turn it into a rollergun, you would have to use different pre-tension settings for the following shafts 6.25mm x 120cm, 6.5mm x 120cm, 6.75mm x 115cm, 7mm x 110cm, 7.5mm x 110cm. The two smaller shafts might be more compatible with a 14mm-14.5mm band with a pre-tension of 50% - 70% elongation, respectively. The 6.75mm and 7mm shafts may work best with a 16mm band with the pre-tension set at 70% elongation and 90% elongation, respectively. And, the 7.5mm shaft might best be paired with a 17.5mm-18mm band at a pre-tension setting of 80%-100% elongation.

Recoil progressively increases from the smaller diameter to the larger diameter shafts as the speargun mass is constant, and, although the band mass increases, it doesn't keep up with the increase in shaft mass. If you were to experiment with all those different shafts and bands, using the same speargun, you will find a sweet spot for each band/shaft combination, but certain shafts will be fully optimized, meaning their performance in speed, distance, and penetration will be far superior or efficient when compared to the others at their sweet spots. This is what you seek, so take notes.

Personal preference plays a role in this, too. So, keep in mind that what is optimal for you, might not be universally optimal, but it will be close!
__________________

Simplicity is the highest form of sophistication. Leonardo Da Vinci

Vegetarian - that's an old Indian word meaning "lousy hunter". - Andy Rooney

Start children off on the way they should go, and even when they are old they will not turn from it. - Proverbs 22:6 (NIV)

KEEP CALM AND CHIVE ON
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Old 12-22-2014, 12:11 PM   #6
VangysWay
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Re: R.A Carbon 110 roller conversion

Quote:
Originally Posted by SpearoTastic View Post
A roller gun has the effective band stretch of a speargun ~25cm - 30cm longer than whatever barrel length you are using.

If you want to have the same penetration at distance as the longer speargun, use a shaft with the same mass as the shaft of the longer speargun.

For example, you are using a 110 cm barrel roYunller gun which should give you performance similar to a 140 cm conventional speargun with a single 18mm band with the same stretch ratio.

Using 40cm as the typical shaft overhang, the shaft mass for your 6.75mm x 140cm shaft is ~390 grams. However, a 140cm speargun is going to use a minimum 7mm diameter shaft. The shaft mass for a 7mm x 180cm shaft is ~540 grams. That is a difference of 150 grams.

To make sure the shaft behaves the same with the additional energy, it needs to have the same mass, which means you should use a 7.5mm x 150cm shaft (~520 grams) or an 8mm x 140cm shaft (~530 grams). Even though these shafts are not an exact match to the mass of the longer shaft, they are close enough and will give downrange performance far beyond the 6.75mm x 140cm shaft.

If you stick with the lighter shaft, you will gain muzzle velocity, but lose penetration at distance because the lighter shaft looses velocity quicker than the heavier shaft due to hydrodynamic frictional forces.

================

The other thing you will have to sort out is the amount of preload to use.

Preloading or pretensioning the band allows the band to provide a force on the shaft throughout the entire powerstroke, much like a pneumatic.

For example, assuming the bands are cut at 3.5:1 stretch ratio, we will first look at the roller without any preload. When the band comes to rest the force on shaft up to the moment before it is arrested is zero. The amount of energy placed into the shaft is (711 Newtons x 1.1 meters)/2 = 391 joules.

Now, if the bands are pretensioned by 100% elongation, the force on the shaft by the band the moment before it is arrested is ~332 N. So, the amount of energy placed into the shaft is [(711 N - 332 N) x 1.1 meters]/2 + 332 N x 1.1 meters = 573 joules..

So, over the same distance, the pretensioned setup provides an additoinal 182 joules of energy.

Please note, that the correct amount of pretension can be very shaft specific, so it varies from shaft to shaft, with larger diameter shafts handling more pretension than smaller diameter shafts, lengths being equal.

I have not come across a shortcut for determining the correct pretension, but have seen the 100% elongation recommended. The best advice I was given was to shorten the cord connecting the band ends to the speargun 1cm at a time until accuracy and/or penetration falls off, then lengthen the cord back to the setting that gave the most accuracy and/or penetration. Tedious, at best, but if you take good notes, you will know where to start on the next roller gun....
Your posts are awesome. Thanks for sharing and keep em coming!
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Old 12-22-2014, 02:50 PM   #7
mishu1984
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Re: R.A Carbon 110 roller conversion

Quote:
Originally Posted by SpearoTastic View Post
Band area is directly proportional to band force, so, yes, band diameter does play a role in pre-tension levels.

Pre-tensioning factors are:

How much you can pull back? If you cannot pull 60-80 lbs from the very top of your speargun, then do not use that much pre-tension.

How much your muzzle can handle? Too much pre-tension and you can cause your muzzle to fail. Remember, the muzzle has to arrest the bands at the pre-tension force, so, to the muzzle, an 80 lb pre-tension is like an 80 lb impact at the end of the powerstroke. Not all muzzles are designed for such a force.

The ratio of the shaft mass/band mass/speargun mass. Pre-tensioning is more critical with a thinner diameter shaft. One can expect the pre-tensioning force to increase with the diameter and mass of the shaft. For example, if you were to take an 80 cm railgun and turn it into a rollergun, you would have to use different pre-tension settings for the following shafts 6.25mm x 120cm, 6.5mm x 120cm, 6.75mm x 115cm, 7mm x 110cm, 7.5mm x 110cm. The two smaller shafts might be more compatible with a 14mm-14.5mm band with a pre-tension of 50% - 70% elongation, respectively. The 6.75mm and 7mm shafts may work best with a 16mm band with the pre-tension set at 70% elongation and 90% elongation, respectively. And, the 7.5mm shaft might best be paired with a 17.5mm-18mm band at a pre-tension setting of 80%-100% elongation.

Recoil progressively increases from the smaller diameter to the larger diameter shafts as the speargun mass is constant, and, although the band mass increases, it doesn't keep up with the increase in shaft mass. If you were to experiment with all those different shafts and bands, using the same speargun, you will find a sweet spot for each band/shaft combination, but certain shafts will be fully optimized, meaning their performance in speed, distance, and penetration will be far superior or efficient when compared to the others at their sweet spots. This is what you seek, so take notes.

Personal preference plays a role in this, too. So, keep in mind that what is optimal for you, might not be universally optimal, but it will be close!
THANK YOU SIR!!!! You should write a book or a column or even start a podcast
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